In the 1990s, an MR (Magneto-Resistive effect) head and GMR (Giant Magneto-Resistive effect) head were put to practical use, and this dramatically increased the recording density and recording capacity of an HDD (Hard Disk Drive). In the 2000s, however, the problem of thermal fluctuation of a magnetic recording medium became conspicuous, and the increasing speed of the recording density temporarily decreased. Still, the recording density of the HDD is recently increasing by about 40% per year because perpendicular magnetic recording more advantageous for high-density recording in principle than in-plane magnetic recording was put to practical use in 2005.
Even when using this perpendicular magnetic recording method, however, it is probably not easy to increase the recording density because the problem of thermal fluctuation becomes conspicuous.
“A microwave assisted magnetic recording method” has been proposed as a recording method capable of solving this problem. In this microwave assisted magnetic recording method, a microwave magnetic field near the resonance frequency of a magnetic recording medium, which is much higher than a recording signal frequency, is locally applied to the medium. Consequently, the magnetic recording medium resonates, and the coercive force (Hc) the magnetic recording medium to which the microwave magnetic field is applied becomes half or less the original coercive force. By superposing a microwave magnetic field on a recording magnetic field, therefore, magnetic recording can be performed on a magnetic recording medium having a higher coercive force (Hc) and higher magnetic anisotropic energy (Ku).
Unfortunately, a large current must be applied to a spin torque oscillation element (STO) as a microwave generation source of microwave assisted recording. Since this generates Joule heat, long-term reliability is difficult to ensure. A necessary electric current increases especially when the magnetic volume (Mst) of an oscillation layer (FGL) is increased in order to increase the microwave magnetic field strength. Accordingly, demands have arisen for decreasing the oscillation driving voltage of the STO, thereby achieving both a strong microwave magnetic field and long-term reliability.